1. Field of the Invention
The invention relates to an actively shielded, comprising a magnet system for generating a uniform magnetic field in an examination zone within an accommodation space of a magnetic resonance apparatus, said magnet system being formed of concentric first and second cylindrical coil systems which enclose the accommodation space, and which have respective magnetic dipole moments that are oppositely directed and substantially equal.
2. Description of the Related Art
An example of an apparatus of the kind set forth is known from DE-A-3 829 175. The magnet system serves to generate a very uniform magnetic field in the examination zone, the magnetic flux density in the examination zone amounting to, for example more than 1 tesla with an inhomogeneity of no more than a few ppm, the external magnetic stray field being as small as possible, for example less than 500 .mu.T at a distance of a few meters from the magnet system. In a magnetic resonance (MR) apparatus of the kind set forth, the external magnetic field generated by the second coil system offers a substantial compensation for the external magnetic field of the first coil system, so that on the one hand the resultant external magnetic field is strongly reduced and on the other hand an external influencing of the stray field hardly has any effect on the uniformity of the field within the examination zone; in this respect see also EP-A-3 138 270 (PHN 10,800). The first and the second coil system preferably consist of superconducting coils.
The cited document DE-A-3 829 175 explains that in order to achieve an acceptable uniformity of the magnetic field in the examination zone, the first coil system should consist of three pairs of coils which are symmetrically arranged relative to the symmetry plane. The second coil system in the known apparatus consists of two pairs of coils which are symmetrically arranged relative to the symmetry plane. The total number of coils required thus amounts to ten. It would be desirable to reduce the number of coils because on the one hand, as explained in DE-A-3 829 175, the manufacturing precision decreases as the number of coils is greater and because on the other hand the manufacturing costs increase as the number of coils is greater.
It is an object of the invention to provide a magnetic resonance apparatus of the kind set forth in which the number of coils required is smaller than in the known apparatus, the magnetic field in the examination zone nevertheless being at least equally uniform. To achieve this, the apparatus in accordance with the invention is characterized in that the first coil system comprises a central coil which is symmetrically arranged relative to the symmetry plane and two pairs of further coils which are symmetrically arranged relative to the symmetry plane, the second coil system comprising a central coil which is symmetrically arranged relative to the symmetry plane and a pair of further coils which is symmetrically arranged relative to the symmetry plane. The invention is based on the recognition of the fact that, when a part of the turns of the second coil system is concentrated in a central coil which is symmetrically arranged relative to the symmetry plane, the number of coils in the first coil system can be reduced to five without causing a loss of uniformity of the magnetic field in the examination zone. The total number of coils in the magnet system can thus be reduced to eight.
It has been found that the further coils of the second coil system are preferably arranged so far outside the symmetry plane that the axial dimensional of the second coil system is substantially equal to that of the first coil system.